1. Field of the Invention
This invention relates to the preparation of metal alloy products by powder metallurgy techniques.
2. Description of the Prior Art
The technique of alloying metals by powder metallurgy has provided a major advance in the manufacture of high performance metals, particularly aluminum-based metals. According to this well known process, a powder or particulate is formed by any of the wide variety of known techniques such as, for example, atomization of various types and rapid solidification technology including ribbon and splat techniques. In general, the particles are formed at such a fast rate that coarse constituents or dispersoids do not have a chance to segregate from the crystal structure. The result is a solid solution containing alloying elements in quantities well above those achievable in products cast in ingots. Consequently, unusually favorable corrosion resistance properties as well as mechanical and other properties are achieved.
Part of the overall process involves the transformation of the powders into solid billets which are capable of being worked and formed as needed in conventional metals processing. Exposure to elevated temperatures during this transformation is generally avoided in an attempt to avoid changes in the crystal structure and attendant losses of superior properties. Porosity must be minimized as well since gas-filled pockets in the final product degrade such properties as toughness, fatigue resistance, ductility, stress corrosion resistance and weld quality.
Porosity in the ultimate product occurs in two ways--by the entrapment of inert gases originally surrounding the powder particles upon closure of the pores, and by the generation of gases during the reaction of certain molecular species with the metal during the processing steps. An example of the latter is chemisorbed and physically bound water at the crystal surfaces reacting with the metal to form a solid oxide, leaving gaseous hydrogen as a by-product.
Accordingly, various procedures have been developed for the removal of pore-forming species from partially compacted ("green") specimens prior to compaction of the specimens to full density.
The process disclosed in Roberts, U.S. Pat. No. 3,954,458 (May 4, 1976) is directed to aluminum alloys specifically, and offers a solution which involves the use of a high vacuum (less than 10.sup.-3 torr) at moderate temperature (450.degree.-850.degree. F.), rather than a moderate vacuum at high temperature (900.degree.-1050.degree. F.). The high vacuum disclosed in this reference requires placing the green compacts in welded aluminum canisters. According to the disclosure, isostatic compaction is used to prepare the green compacts before placement in the canisters. Once a compact is in the canister, the high vacuum is drawn (at the moderate temperature) and the canister is sealed to retain the vacuum. Compaction to full density is then achieved by crushing the entire canister with compact sealed inside at a pressure of 133 ksi. The canister must then be removed by scalping. Both the canning and scalping processes are labor-intensive and therefore costly.
An improvement over this process is disclosed in Roberts, U.S. Pat. No. 4,104,061 (Aug. 1, 1978). This improvement is directed to powder metallurgy alloys in general, and it addresses the length of time required for the degassing step, as well as the danger of porosity regeneration in the compacted product during subsequent exposure to high temperatures. The improvement involves the purging of the green compact with a "depurative" gas prior to final compaction. A depurative gas is one which mixes with volatilized species originally bound to the surface of the metal (such as water molecules), and thereby helps remove or "wash" the volatile contaminants out of the green compact during subsequent evacuation. The preferred such gases are those which also react with either the metal matrix or the alloying elements during the final densification or working to produce reaction products which are entirely solid. Accordingly, these preferred gases are commonly referred to as reactive gases. In order to minimize the amount of these reaction products present in the ultimate product, the reactive gas is still evacuated at moderate vacuum according to this disclosure, requiring the use of the canister as before. Therefore, while this disclosure provides improvements in both processing time and ultimate product stability, the expense of the canisters and their removal is still present.
An alternative method of removing pore-forming species is disclosed in Hildeman et al., U.S. Pat. No. 4,435,213 (March 6, 1984). This disclosure is directed to the removal of chemically bonded water molecules from a green compact. Rather than heating the compact under a high vacuum, the process uses rapid induction heating. Even then, however, the process is only of use where toughness is not a concern. For maximum toughness, the patentees state that evacuation of the green compact is still needed.
In all cases, the green compact is formed by isostatic compression of the powder at ambient temperature prior to removal of the pore-forming species. Such removal is achieved by the use of high temperature and high vacuum for prolonged periods, the combination of moderate temperatures, moderate vacuum and depurative gas for shorter periods, or the use of induction heating whether under vacuum or not. Isostatic compression is done primarily for ease of handling, and generally stops short of sealing off the internal pores, leaving a free passage from the pores to the exterior of the compact to permit the escape of gases. Either induction heating or high vacuum degassing in sealed canisters is then used to minimize both porosity and the amount of solid reaction product in the ultimate product. For maximum tensile properties, final compression to full density is then done on these open pore compacts while the latter are still under high vacuum.